An inkjet recording system has been widely used at present from the reasons that high-speed recording is achieved, colorization is easy, a regular paper can be used as a recording member, and the like.
As an inkjet ink (hereinafter sometimes simply referred to as an ink) for use in this inkjet recording system, a dye ink and a pigment ink are mentioned. In recent years, the pigment ink which is excellent in weather resistance, color density, generation of bleeding, and the like has been widely used. In particular, in combination with the spread of digital cameras, a demand in the photograph printing field requiring high print density (coloring properties) has increased due to the height of the fastness of a printed product.
Since pigments for use in this pigment ink are basically insoluble in aqueous media and organic media, the ink or the pigment dispersion is obtained as one in which the pigments are dispersed in the media.
Therefore, the ink and the pigment dispersion have had a problem in that the pigments aggregate or the like in a long-term storage, so that it is difficult to maintain the dispersibility for a long period of time and stably store the same. With respect to the dispersion stability for a long period of time, a large number of measures for maintaining the dispersion stability, such as use of a specific polymer composition and modification of the pigment surface with a specific polymer, have been proposed (e.g., Patent Documents 1 and 2). For example, in a polymer composition described in Patent Document 1, the dispersion stability is excellent and also the gloss of a photograph printed product printed on a photograph paper can be increased similarly as in a silver halide photograph.
On the other hand, the dispersion stability of the pigment is improved to some extent by the use of a polymer described in Patent Document 2. However, the gloss, the print density (coloring properties), the fixability (abrasion properties), and the like, for example, of a printed product are affected in some cases and it cannot always satisfy a demand required in a printed product with high image quality in recent years. Moreover, the pigment surface is required to be modified with the polymer described above, and therefore there has been room for an improvement also in terms of the productivity.
When manufacturing the pigment dispersion and the inkjet ink, removal of particles of a size equal to or larger than a fixed size which may be contained in the pigment dispersion or the inkjet ink has been generally performed. As such treatment, filter filtration, centrifugal separation, and the like are mentioned. In the case where the number of particles to be removed is large, an exchange frequency of a filter and a removal frequency of particles removed by centrifugal separation have become high, so that the productivity of the pigment dispersion and the inkjet ink has decreased in some cases.